The goal of this project is to identify and evaluate factors that may influence variability in response in laboratory studies. For example, following a detailed histopathological examination, we examined the lobe-specific incidences and degrees of severity of background prostatic, seminal vesicular, and ampullary glandular lesions in 1768 control Fischer 344 rats from 35 recent National Toxicology Program (NTP) two-year rodent bioassays carried out at four different laboratories. Inflammation in the dorsolateral lobes was significantly correlated with pituitary gland adenoma, and prolactin was suggested to play an important role in the pathogenesis of prostatic inflammation. Significant lab-to-lab variability in the amount of tissue sampled was also observed, and we suggested an optimal embedment and trimming method in rat prostate and seminal vesicle to ensure adequate and consistent sampling. In another project, we investigated the factors affecting inter-scorer and inter-laboratory variability in response in the mouse epididymal sperm aneuploidy (mESA) assay. We identified microscope scoring criteria as the major source of interlaboratory variation, which emphasizes the importance of strict technical controls for this assay. Several years ago the NTP developed and adopted a new rodent diet (NTP-2000), a diet higher in fiber and lower in protein than the previously used NIH-07 diet. Data are accumulating which will allow the comparison of tumor incidence, body weight, and survival in animals fed this new diet with animals fed the NIH-07 diet. This ongoing investigation will help the NTP decide whether or not additional measures such as dietary restriction are needed in its long term rodent bioassays to reduce body weight to an acceptable level and to lower background tumor rates.